The present invention relates to a bifocal optical system, having a long focal length LF and a short focal length SF and comprising an objective with fixed lenses arranged in a front group and a rear group, and an intermediate group in a movable holder for insertion between the front and rear groups.
An optical system of this type, having three groups of lenses, is disclosed in U.S. Pat. No. 4,449,791. In this Patent, the optical system is of the zoom objective type, that is to say that the variation in focal length is obtained by moving second and third groups along the optical axis. In a bifocal optical system of this type the long and the short focal length can be obtained by moving one single group, which has the advantage of a structure of small bulk. However, the choice of such a solution is based on the presumption of very strict tolerances in the positioning of the movable holder to obtain an adequate stability of the axis of the long focal length and preventing a lateral shift of the image in the focal plane due to shifts of the holder, which renders such a realisation delicate and difficult. A less difficult solution to preserve an adequate stability of the axis of long focal length is to determine the long focal length only by fixed lenses, whilst passing to the short focal length is obtained by rotating the holder around an axis which is perpendicular to the optical axis. Such a solution is proposed in the publication SPIE, vol. 131, Practical infrared optics (1979), p. 24, but this solution has the disadvantage that it is particularly complicated.